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demonstrate an understanding of the terms: perihelion, aphelion,
greatest elongation, conjunction, opposition, transit and occultation
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We know that the orbits of planets around the Sun are not circular, but are elliptical.
This was stated by the astronomer Kepler in his first law. As a result, during one
orbit, there must be a point where the Earth (or other planet) is closest to the Sun
and another point where the planet is furthest away.
The following diagram shows this :-
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PERIHELION
CLOSEST TO THE SUN
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PLANET
SUN
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APHELION
FURTHEST AWAY FROM THE SUN
If the orbit of the Earth was circular, the mean Earth to Sun distance would always be
the same. This distance is called 1 Astronomical Unit (1AU)
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1AU = 150,000,000 km
Picture credits : (Sun) SOHO/ESA&NASA (Earth) NASA/JSC-Apollo17
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GREATEST ELONGATION
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This is the largest angular distance of Mercury or Venus from the Sun.
This is only for the planets that orbit the Sun inside the orbit of the Earth.
Greatest elongation is the best time to view these inner planets as they will be seen at
their furthest angular distance from the Sun, well away from a line of sight with the
Sun.
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Inferior planets (orbit between the Earth and the Sun)
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These will move across the sky, always being relatively close to the Sun.
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Mercury is only seen:-
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! Before sunrise After sunset
When the Sun is present in the sky, the glare from the Sun means the planet can no
longer be seen.
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As Venus has a larger orbit, it can be seen further away from the Sun either:-
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Before or after dawn Before or after dusk
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VENUS
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MERCURY
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SUN
(BELOW HORIZON)
Viewing Mercury !
Mercury can only be seen at a few times through the year, as it is often in a line of
sight close with the Sun and impossible to see as a result. In the evening, about 30
to 45 minutes after sunset, or in the morning about 30 to 45 minutes before sunrise
– these are the only times you will be able to see Mercury when it is near to greatest
elongation. Once the Sun is visible, the glare prevents Mercury being seen. The path
that Mercury tracks across the sky from the horizon is similar to the path shown
below (as for May 2014):-
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May 25 – GREATEST ELONGATION (evening)
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Horizon
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May 10
May 30
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Tip: Stand with your arm out straight and your thumb
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up. If you are trying to see Mercury, it will never rise
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above the tip of your thumb, with the base of the
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thumb level with a clear, low horizon.
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Viewing Venus
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Venus will usually be higher in the sky than Mercury because it has a bigger orbit.
Venus can be seen as the bright ‘evening star’ towards the west, or the bright
‘morning star’ towards the east, being visible to magnitude - 4.5. This makes Venus
one of the easiest objects to see as it can be extremely bright and stays in the sky
over months at a time. Venus can be a considerable distance from the Sun in the sky
and so the planet can appear in the sky even with the Sun visible.
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Diagram showing the position of the inferior planets at greatest elongation
Planet in the most favourable position to be observed
(Mercury or Venus)
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View from
Earth
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θ = 22˚ for Mercury !
θ = 47˚ for Venus
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θ is the angle for greatest elongation of either Mercury or Venus
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Picture credit :
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(Sun) SOHO/ESA&NASA (Earth) NASA/JSC-Apollo17
The position of the planet in its orbit means that for each complete orbit there are 2
greatest elongations. One is as shown in the diagram and the other is when the
planet is on the opposite side of the Sun. One gives the opportunity of seeing the
planet after the Sun has set (greatest eastern elongation) and the other allows the
planet to be seen before dawn (greatest western elongation). Dates for future
greatest elongations are:-
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MERCURY
VENUS
November 18
November 1
May 25
March 22
June 25
June 6
September 4
October 26
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Conjunction
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Conjunction is when 2 celestial objects are in line when viewed from Earth eg when a
planet is in line with the Sun – inferior planets could be in front or behind the Sun,
whereas superior planets (planets orbiting the Sun outside Earth’s orbit), can only be
behind the Sun. The elongation angle of a planet at conjunction is 0˚. The planet
cannot be seen because the Sun’s bright light inhibits its view (except at a time of
transit for the inferior planets, where special techniques are used for viewing).
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Earth
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! Outer Planet
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Superior
! Conjunction
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Picture credit :
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Sun
Inner Planet
Inferior
Conjunction
Inner Planet
Superior
Conjunction
(Sun) SOHO/ESA&NASA (Earth) NASA/JSC-Apollo17
Superior conjunctions are when the planet is on the far side of the Sun from the
Earth. Looking across the distance of the majority of an orbit, a planet will look
small at times close to conjunction, when the planet is on the far side of the Sun.
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Inferior conjunctions can only occur with the inferior planets which have their orbit
inside the orbit of the Earth, where Mercury or Venus pass on the near side of the
Sun. When an inferior planet moves directly across the line of the Sun, a transit, the
planet can be seen (but only using specialist equipment AND NEVER WITH THE EYES
LOOKING DIRECTLY AT THE SUN OR THROUGH AN OPTICAL INSTRUMENT). !
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Opposition
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Opposition is when the Sun, the Earth and an outer planet are in line again, but we
look away from the Sun at night to the outer planet. The planet can appear very
bright because:-
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so the outer planet appears quite large
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directly back to Earth :-
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Magnitudes:
Mars -2.8
Jupiter -2.5
Saturn -0.3
Uranus 5.5
Opposition can only occur for the outer planets, as the inner planets will always be in
the direction looking towards the Sun. Viewing of the outer planets is best at
opposition.
Earth
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Sun
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Outer Planet
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Opposition
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Picture credit :
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Dates for future oppositions of the outer planets are:!! MARS
JUPITER
SATURN
URANUS
NEPTUNE
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January 5
April 28
October
August 26
2014
2013
2013
2013
!! 2014
May 22
February 6
May 10
October
August 29
!! 2016
2015
2014
2014
2014
27
March 8
May 23
October
September 1
!! July2018
2016
2015
2015
2015
!! October 13
April 7
June 3
October
September 2
2020
2017
2016
2016
2016
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(Sun) SOHO/ESA&NASA (Earth) NASA/JSC-Apollo17
2 years 7 weeks
1 year 1 month
1 year 2 weeks
1 year 4 days
(Time between successive oppositions)
1 year 2 days
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Transit
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The route of a smaller celestial body passing in front of a larger one eg Mercury or
Venus in front of the Sun - or a moon in front of its planet eg Io in front of Jupiter. !
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Picture credit :
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(Sun) SOHO/ESA&NASA
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A view of the transit of Venus on 8
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June 2004
Venus can clearly be seen passing in front of the Sun at about halfway through the
transit. The photograph is of the image which had been projected on to a screen,
using a pair of binoculars.
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NEXT MERCURY TRANSIT (UK)
May 9
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NEXT VENUS TRANSIT
December 11
(not visible in UK)
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Occultation
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The route of a celestial body passing behind another one eg when a planet blocks the
view of its moon (Io passing behind Jupiter). Other examples are when a planet
disappears behind the moon, a star disappears behind a planet, or an asteroid
disappears behind the moon. Occultations are common events.
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Picture credit : (Moon) NASA-JPL
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!!The sequence of pictures below show the! occultation of Jupiter behind the moon
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Picture credit : Becky Coretti and Bill Williams